Websocket support was introduced with Conga v3.0. By default, MiServer v3.0 uses whatever version of Conga is installed with your current Dyalog APL (it copies the DRC namespace from the Conga workspace, unless you've saved the MiServer workspace with a preexisting DRC). So, in theory, you have the means at your disposal to build a framework to use websockets, but I wouldn't recommend doing so - it's not a simple task.

We plan to release MiServer v3.1 this year which uses Conga v3.0's built-in HTTP support and will provide a websocket widget and management framework. MiServer v3.1 is currently in the development branch of the MiServer GitHub repository. We have not yet added websocket support, but that's on the short list of things to work on next.